1. Field of the Invention
The present invention relates to a vertical deflection driving circuit applicable for a television receiver and various kinds of display instruments.
2. Description of the Related Art
A vertical deflection driving circuit according to the conventional example is shown in FIG. 3. Sawtooth waveform voltage Vsaw for vertical deflection applied to an input terminal 1 is supplied to inversion input terminal (xe2x88x92) of a differential amplifier circuit 2 through a resistor R0. To non-inversion input terminal (+) of the differential amplifier circuit 2, reference voltage Vref is supplied.
A capacitor C3 for pumping-up and a diode D1 construct a pumping-up circuit. While scanning period, a terminal X of the capacitor C3 for pumping-up is charged to power source voltage Vcc through the diode D1 by switching a switch SW to a terminal 3 side. On the other hand, while retracing period, the terminal X of the capacitor C3 for pumping-up becomes 2Vcc owing to capacitor coupling by switching the switch SW to a terminal 4 side. Here, forward threshold voltage Vf of the diode D1 is neglected. Voltage of the terminal X being output of the pumping-up circuit is supplied as power source voltage of the differential amplifier circuit 2.
Then, output of the differential amplifier circuit 2 is supplied to one terminal of a vertical deflection yoke coil DY. The other terminal of vertical deflection yoke coil DY is grounded through series circuit consisting of a capacitor C1 and a resistor R1. Voltage of connecting point B between the capacitor C1 and the resistor R1 is fed back to the inversion-input terminal (xe2x88x92) of the differential amplifier circuit 2 through a resistor R2. An integrating circuit consisting of resistors R3 and R4 and a capacitor C2 carries out S-correction by integrating voltage of the other terminal of the vertical deflection yoke coil DY, and the voltage being carried out the S-correction is fed back to the inversion-input terminal (xe2x88x92) of the differential amplifier circuit 2.
Waveforms at each main point of the vertical deflection driving circuit of the above-mentioned construction are shown in FIG. 4. In FIG. 4(A), deflecting current i at point A in FIG. 3 is shown. Current changes largely at boundary of retracing 20 period Tr and scanning terminal Tt. By the large change of current, counter-electromotive force generates at the vertical deflection yoke coil DY.
By detecting that voltage generating at this time is higher than reference voltage Vref (usually equal to power source voltage Vcc supplied to the vertical deflection driving circuit) and switching the above-mentioned switch SW, the pumping-up circuit is made on and the driving voltage is made 2Vcc only while retracing period.
Thus, the driving voltage at point A in FIG. 3 is 2 Vcc as showing in FIG. 4(B). In FIG. 4(C), voltage waveform (ixc3x97R1) at point B in FIG. 3 is shown. The voltage is fed back to the inversion-input terminal (xe2x88x92) of the differential amplifier circuit 2 through the resistor 2.
Now, a vertical deflection driving circuit of PWM method as shown in FIG. 5 is known. This method is fundamentally a circuit where Pulse Width Modulation is carried out to input sawtooth waveform voltage and further the voltage is restored to its original sawtooth waveform through a filter 14 consisting of a coil L and a capacitor C. In FIG. 5, sawtooth voltage Vsaw for vertical deflection is supplied to non-inversion input terminal (+) of a comparator 10 from an input terminal 11.
On the other hand, sawtooth wave of the designated frequency f is supplied to an inversion-input terminal (xe2x88x92) of the comparator from an input terminal 12 as a carrier signal. Then, to output 13 of the comparator 10, a PWM signal having pulse width according to level of input sawtooth voltage Vsaw is obtained as shown in FIG. 6. The PWM signal is restored to waveform of input sawtooth wave by the filter 14 (low-pass filter) consisting of the coil L and the capacitor C.
As described above, in the conventional vertical deflection driving circuit, retracing period is detected by counter-electromotive force generating at the vertical deflection yoke coil DY and vertical deflection driving is carried out by making the pumping-up circuit on.
However, when the vertical deflection yoke coil L is driven by the above-mentioned vertical deflection driving circuit of PWM method, counter-electromotive voltage Va of the power source Vcc 1 side and counter-electromotive voltage Vb of the ground GND side generate at each pulse as shown in FIG. 6. Here, FIG. 6(A) shows a waveform of a carrier signal and FIG. 6(B) shows a waveform of a PWM signal.
Because of that, there is a problem that malfunction occurs in the pumping-up circuit and stable vertical deflection driving can not be carried out in the method that retracing period is detected by counter-electromotive force generating at the vertical deflection yoke coil DY and the pumping-up circuit is made on as the conventional circuit.
Then, an object of the invention is to prevent malfunction of the pumping-up circuit and to enable to drive vertical deflection stably by removing influence of counter-electromotive voltage while scanning period in the vertical deflection driving circuit of PWM method.
Further another object is to provide a circuit detecting retracing period stably and operating the pumping-up circuit instead of conventional method detecting retracing period by counter-electromotive force.
Then, the invention is a vertical deflection driving circuit characterized in having a pre-amplifier receiving sawtooth waveform voltage for vertical deflection, a first comparator pulse-width-modulating output signal of said pre-amplifier by comparing a predetermined carrier signal and output signal of said pre-amplifier, a transistor circuit for driving supplied with output signal of the first comparator, a pumping-up circuit pumping-up power source voltage supplied to said transistor circuit for driving while retracing period, and a vertical deflection yoke coil supplied with output signal of said transistor circuit for driving, and a voltage absorbing circuit absorbing counter-electromotive voltage generating from said vertical deflection yoke coil while scanning period is provided.
According to the construction, it is possible to prevent malfunction of the pumping-up circuit and to carry out stable vertical deflection driving.
The voltage absorbing circuit comprising a diode connected between source and drain of said transistor circuit for driving, a capacitor for pumping-up of said pumping-up circuit, and a transistor for pumping-up switching in which drain thereof is connected to one end of said capacitor for pumping-up, source thereof is grounded, and for charging said capacitor for pumping-up making the transistor on while said scanning period, and said counter-electromotive voltage is absorbed to a ground line through said diode, said capacitor for pumping-up, and said transistor for pumping-up switching.
According to the construction, as the circuit is constructed so as to absorb counter-electromotive voltage generated by the vertical deflection yoke coil using the capacitor for pumping-up and the transistor for pumping-up while scanning period, it is possible to depress increase numbers of circuit elements to the most.
Further adding the above-mentioned construction, a second comparator comparing output signal of said pre-amplifier and reference voltage to detect retracing periods and outputting detecting signal and a pumping-up switching circuit operating said pumping-up circuit responding on detecting signal of said second comparator are provided in the circuit. According to this, it is possible to detect retracing period and to operate the pumping-up circuit without using counter-electromotive generated by the vertical deflection yoke coil.